Measurements of the decay constant require good (short) time resolution at early
times (t small, I large) when the changes in gamma-exposurerate are most rapid.
At
later times (t large, I small), the rate of change of the gamma-exposure rate of the
gamma radiation is much smaller, and the instrument system need not have such good
time resolution. The instrument for the measurement of residual-gammaradiation was
designed to cover a range from 1 r/hr with a time resolution of 5 minutes, to 10‘ r/ar
with a time resolution of 0.05 minutes. The basic circuit is shown in Figure 2.1, where
ly
nN
>
>
|
1
i
:
22%5V
—
i SENSING
| ELEMENT |
4
-
oy
—~——
I
—~
Cy
poe cree
22> V
41 4
|
100K S
OUTS
Figure 2.1 Schematic diagram showing the basic circuit for the Conrad and Gustave
detectors.
The Conrad detector used an unsaturated ion chamber as the sensing
element, whereas the Gustave detector used a scintillation detector.
the sensing elementis an unsaturated ion chamber.
The ion chamber was designed to
have a current output proportional to the square root of the gamma-~exposure rate.
overall detector response is given by:
f= kr?
The
(2.2)
Where: f = the output frequency
r = the gamma-exposure rate in r/hr
k = a parameter chosen to meet specific design objectives
In laboratory calibrations on a 250-kv X-ray beam, these detectors showed a preci-
20